IME, they are not as robust a grower as hardy geraniums with the more standard green foliage and depending on selection, will lose some of their coloring as the season progresses. And you will definitely see a muddying or washing out of color in such a small amount of sun. Personally, I think dark leaved plants need a bright sunny location to offer both their best foliage effect as well as their best contribution to the garden composition - dark leaved plants tend to disappear in low light conditions.